java setBackground第一次不起作用，但第二次起作用，为什么？

Question

我在一本书中看到过这个例子，效果很好，但出现的唯一问题是，当clearCounter变为==5时，第一次调用绘制时背景不会变黑，然后屏幕会被清除，当绘制开始时，背景会变黑

public class apletprg extends JApplet implements ActionListener 
    {
int clearCounter;
    Timer t;
public void init(){

    setBackground(Color.black);
    clearCounter = 0;
    Timer t = new Timer(1000, this);
    t.start();

}

public void paint(Graphics g)
{
    setBackground(Color.black);
    clearCounter++;
    Graphics2D g2 = (Graphics2D) g;

    if (clearCounter == 5){
        g2.clearRect(0, 0, 500, 400);
        clearCounter=0;
    }

    for (int i = 1; i <= 40; i++) {
        Color c = chooseColor();
        g2.setColor(c);
        Font f = chooseFont();
        g2.setFont(f);
        drawJava(g2);
        }
}
public void actionPerformed(ActionEvent ae){
repaint();
}
public Color chooseColor(){
int r= (int) (Math.random() * 255);
int g= (int) (Math.random() * 255);
int b= (int) (Math.random() * 255);
Color c = new Color(r,g,b);
return c;
}
public Font chooseFont(){

    int fontChoice = (int) (Math.random() * 4) + 1;
    Font f = null;
    switch (fontChoice) {
    case 1: f = new Font("Serif", Font.BOLD + Font.ITALIC, 20);break;
    case 2: f = new Font("SansSerif", Font.PLAIN, 17);break;
    case 3: f = new Font("Monospaced", Font.ITALIC, 23);break;
    case 4: f = new Font("Dialog", Font.ITALIC, 30);break;
    }
    return f;

}
public void drawJava(Graphics2D g2){
    int x = (int) (Math.random() * 500);
    int y = (int) (Math.random() * 400);
    g2.drawString("Adnan", x, y);   
}
}

我知道Init（）在开始时只被调用一次，但为什么它不能在开始时更改背景呢

Answer 1

解决了这个问题，只需再添加一个变量，并在绘制开始时调用ClearRect（），并通过新添加的变量确保只调用一次

public void init（）{

    setBackground(Color.black);
    clearCounter = 0;

    Timer t = new Timer(1000, this);
    t.start();
    check = 0;   <       New Variable


}
public void paint(Graphics g)
{
    if (check==0){
        g.clearRect(0, 0, 500, 400);   <       To Ensure That it will Excute Only Once , beacuse check is incremented later in Code
    }



    clearCounter++;
    check++;
    Graphics2D g2 = (Graphics2D) g;

    if (clearCounter == 5){
        g2.clearRect(0, 0, 500, 400);
        clearCounter=0;
    }

    for (int i = 1; i <= 40; i++) {
        Color c = chooseColor();
        g2.setColor(c);
        Font f = chooseFont();
        g2.setFont(f);
        drawJava(g2);
        }
}

Answer 2

在init方法中，将setBackground(Color.black)替换为getContentPane().setBackground(Color.black)

并在paint方法中添加super.paint(g)作为第一行

否则，如果您不想使用Swing特性，那么继续导入java.applet.Applet，并使您的类扩展Applet，而不是JApplet

public class NewClass extends JApplet implements ActionListener {

    int clearCounter;
    Timer t;



    public void init() {

        getContentPane().setBackground(Color.black);
        repaint();
        clearCounter = 0;
        //t = new Timer("1000", true);


    }

    public void paint(Graphics g) {
        super.paint(g);
        setBackground(Color.black);
        clearCounter++;
        Graphics2D g2 = (Graphics2D) g;

        if (clearCounter == 5) {
            g2.clearRect(0, 0, 500, 400);
            clearCounter = 0;
        }

        for (int i = 1; i <= 40; i++) {
            Color c = chooseColor();
            g2.setColor(c);
            Font f = chooseFont();
            g2.setFont(f);
            drawJava(g2);
        }
    }
    @Override
    public void actionPerformed(ActionEvent ae) {
        repaint();
    }

    public Color chooseColor() {
        int r = (int) (Math.random() * 255);
        int g = (int) (Math.random() * 255);
        int b = (int) (Math.random() * 255);
        Color c = new Color(r, g, b);
        return c;
    }

    public Font chooseFont() {

        int fontChoice = (int) (Math.random() * 4) + 1;
        Font f = null;
        switch (fontChoice) {
        case 1:
            f = new Font("Serif", Font.BOLD + Font.ITALIC, 20);
            break;
        case 2:
            f = new Font("SansSerif", Font.PLAIN, 17);
            break;
        case 3:
            f = new Font("Monospaced", Font.ITALIC, 23);
            break;
        case 4:
            f = new Font("Dialog", Font.ITALIC, 30);
            break;
        }
        return f;

    }

    public void drawJava(Graphics2D g2) {
        int x = (int) (Math.random() * 500);
        int y = (int) (Math.random() * 400);
        g2.drawString("Adnan", x, y);
    }
}

如果你想执行超级。paint（）只需在类中添加一个布尔变量一次

boolean firstTime = true;

在油漆中

if(firstTime) {
    super.paint(g);
    firstTime = false;
}